1. Field of the Invention
The present invention relates to an absolute encoder for detecting the absolute position of a measurement subject.
2. Description of the Related Art
Absolute encoders are used in the field of machine tools, robots, and the like in order to accomplish highly precise positioning control. An absolute encoder includes, for example, a scale having a light-dark optical pattern, a light emitting element for irradiating the scale with light, a light receiving element for detecting light that has been transmitted through or reflected by the scale, and an arithmetic device disposed in the downstream of the light receiving element, and detects the absolute angle of the scale joined to a rotational axis of a motor or the like.
This type of absolute encoder generally has on the scale an absolute pattern, which is made up of angle-specific patterns for detecting a rough absolute angle, and an equally spaced incremental pattern for enhancing the resolution. Structured as this, the absolute encoder is capable of detecting the absolute angle at high resolution.
However, the improvement in resolution is making heretofore ignored errors non-negligible, and the importance of more precise detection methods is growing.
Heretofore, there have been proposed Methods as disclosed in U.S. Pat. No. 8,759,747 and Japanese Patent Application Laid-open No. 2013-96757 as methods with which high precision detection is accomplished.
In U.S. Pat. No. 8,759,747, for example, an absolute rotary encoder includes a rotating cylindrical body with a plurality of marks arranged on a cylindrical surface along the circumferential direction in fixed cycles, a light source for emitting light to the cylindrical surface, a detector for detecting the marks by way of a plurality of photoelectric conversion elements arranged at a pitch smaller than the cycle of the marks, and a calculation unit for calculating the absolute angle based on an output of the detector. The calculation unit uses correction data to correct a distortion error due to the geometric arrangement of the cylindrical surface and the detector in relation to each other.
In Japanese Patent Application Laid-open No. 2013-96757, a displacement detecting device includes a scale that has a scale pattern including incremental components, an optical system for forming an image of the scale pattern with light, a light-receiving element array for detecting the formed scale pattern image, and an arithmetic circuit for analyzing the position of the scale based on a signal of the light-receiving element array. The displacement detecting device removes distortion of the optical system by virtually rearranging the light receiving elements based on a distortion table, which is obtained from distortion information of the optical system.
However, U.S. Pat. No. 8,759,747 and Japanese Patent Application Laid-open No. 2013-96757 have the following problem:
The absolute rotary encoder of U.S. Pat. No. 8,759,747 corrects the effect of the cylindrical surface for each position of the detector, and can therefore eliminate the effect of the cylindrical surface. However, there is a problem in that, because reducing the cycle of the marks for the purpose of enhancing the resolution gives different widths to a light portion and dark portion of a mark, which is made up of a light portion and dark portion of a received optical signal, due to the light diffraction phenomenon, the precision is not improved by correction for each position of the detector alone.
In the displacement detecting device, as well as a displacement detecting method and a displacement detecting program, of Japanese Patent Application Laid-open No. 2013-96757, the distortion of the optical system is corrected for each position of the detector and deterioration in precision due to the distortion of an image forming lens can therefore be reduced. However, Japanese Patent Application Laid-open No. 2013-96757 has the same problem as U.S. Pat. No. 8,759,747 in that, because reducing the cycle of the marks for the purpose of enhancing the resolution gives different widths to a light portion and dark portion of a mark, which is made up of a light portion and dark portion, due to the light diffraction phenomenon, the precision is not improved by correction for each position of the detector alone.